1. Field of the Invention
This invention concerns weight measuring apparatus. More particularly, the invention is concerned with a weighing scale system having an improved analog-to-digital conversion means including a microcomputer which forms an integral part thereof and to the automatic tracking of the net zero indication of such an apparatus.
2. Description of the Prior Art
Weighing and computing scales must meet several stringent requirements for performance and cost. The scales must be accurate enough to satisfy public weights and measures authorities, yet be available at a reasonably affordable price and perform their operations within a period of time which is convenient for sales transactions.
One of the important factors in digital scales upon which cost, accuracy and operation time depends, is the conversion of the unknown analog weight signal corresponding to an article weight to digital data representative of the article weight.
In the past, digital weighing and computing scales have typically performed the analog-to-digital conversion with a separate distinct and independently controlled analog-to-digital converter circuit which may provide its digital output to a data processing means. Typical examples are Williams, Jr. et al, U.S. Pat. No. 3,709,309 and Loshbough et al., U.S. Pat. No. 3,962,569. This prior art circuitry conventionally uses the dual slope method of analog-to-digital conversion, which method is illustrated in Gilbert, U.S. Pat. No. 3,051,939 and Ammann, U.S. Pat. No. 3,316,547. Triple slope analog-to-digital converters are shown in U.S. Pat. Nos. 3,577,140 Aasnacs; 3,582,947 Harrison; 3,678,506 Wheable; and Re 28706 Dorey. Reference is also made to U.S. Pat. No. 3,937,287 granted to E. G. Pryor and R. C. Loshbough on Feb. 10, 1976 relating to data filtering. The exemplary embodiment of the invention described herein describes features shown in U.S. Pat. Nos. 3,962,569 Loshbough et al; 3,962,570 Loshbough et al; 3,986,012, Loshbough et al; 4,004,139, Hall et al; and application Ser. No. 729,911, Hall et al now U.S. Pat. No. 4,159,521.
Prior art weight measuring apparatus has generally required circuitry separate from the data processing means for performing the integrating, counting, and control functions normally associated with an integrating type analog-to-digital conversion. Attendant with the requirement for separate circuitry is its cost and the relative inflexibility due to the limited number of functions performed by hardwired circuitry.